Why SOMETIMES (!) Do not open some sites?

Question

Computer networks

Subject. After changing the computer SOMETIMES, English-language sites (Twitter, epicnpc, reddit, imgur, puush ​​and others) do not open. Sometimes they open, but not completely, but, for example, only text.
Everything works fine through a proxy. I
checked for viruses. The hosts are clean. After changing the computer, the modem settings did not change.
What could be the problem?

Answer 1

If through the proxy is normal, most likely the problem is with the provider.

Answer 2

In DNS, specify manually for example 8.8.8.8 and 8.8.4.4 and check if the problem persists.